PHOTOLITHOGRAPHY At CNF From Computer Aided Design (CAD) to Patterned Substrate Garry J. Bordonaro Adapted from work by Graham M. Pugh At CNF, many options exist for producing patterned substrates, but deciding which options are best for your application requires considerable planning. Choosing the most appropriate lithography tool and technique depends upon what processes you will perform after exposure. The purpose of this manual is to provide you with the information necessary for you to design the best process for achieving the desired results. Introduction to Nanofabrication ..................................................... pg.- 2 Photolithography Explained ........................................................... pg.- 7 Optical Lithography - Techniques ................................................ pg.- 13 Optical Lithography - Exposure Tools ......................................... pg.- 24 Optical Lithography - Mask Making ........................................... pg.- 53 Process Notes .................................................................................. pg.– 61 1 Introduction to Nanofabrication A Brief History Transistors The first working transistor was demonstrated in 1947 at Bell Labs by researchers Bardeen, Brattain, and Shockley. This device was fabricated in Germanium and was rather large by today’s standards: The First Transistor Courtesy Lucent Technologies Transistors were manufactured as discrete devices beginning in the 1950’s and continue to be produced and used in that and many other forms. The invention of the transistor revolutionized electronics by enabling smaller, lighter, cooler, cheaper, and more reliable products to be produced. It was the beginning of the end for vacuum tubes and the birth of portable consumer electronics. 2 Integrated Circuits The first integrated circuit was demonstrated in 1959 at Texas Instruments by Jack Kilby. The First Integrated Circuit Courtesy Texas Instruments At almost the same time, Robert Noyce was demonstrating a similar device at Fairchild Camera. Noyce’s most important contributions to the future of IC fabrication were the use of planar technology, where all structures of the device are flat and in the plane of the substrate, and the use of silicon dioxide as an insulating material grown on a silicon substrate. These innovations led to the development of nearly all the electronic products existing today. Manufacturing of integrated circuits began in the 1960’s and continues to grow nearly exponentially. 3 Automation The cost-effective manufacturing of modern IC’s began in the 1980’s with the advent of automated control of processing equipment. Wafer Manufacturing Facility Courtesy IBM Previously, semiconductors were made on a scale reminiscent of laboratory experiments. Large-scale manufacturing was only made possible with the introduction of computer-controlled automation. As more and more aspects of the fabrication process were automated, higher yields and efficiencies were achieved; thus, smaller and cheaper devices were made possible. This continuous improvement of wafer processing through the statistical controls available with automated systems helps drive the race to higher performance products we benefit from today. 4 Applications The international marketplace has driven the demand for faster, smaller, and cheaper devices in almost all applications. The first breakout product was the Personal Computer, and while this remains an important customer of IC’s, the real growth areas are many: smartphones, automobiles, wireless networks, and personal entertainment to name a few. Even household appliances like thermostats and refrigerators are utilizing more and more electronic devices as size, cost, and power requirements are reduced, and capabilities and reliability improve. The Future What does this mean to Me? The technologies used to produce these devices were developed almost entirely within and for the electronics industry. Only recently have groups outside the realm of electronics considered applying these methods to constructing other devices. One of the most notable has been biologists, who now are making great strides by applying Nanotechnology to Biology; thus, Nanobiotechnology, which is an important and growing area of research. Courtesy Cornell Nanobiotechnology Center 5 More researchers and industries are discovering new applications each year, and the National Science Foundation sees this as a trend worth supporting. The CNF is a part of the National Nanotechnology Coordinated Infrastructure (NNCI), which is an integrated partnership of nearly 30 associated user facilities supported by NSF, providing unparalleled opportunities for Nanoscience and Nanotechnology research. NNCI Locations Our purpose is to support researchers by providing access to state-of-the-art tools and technologies, combined with our experience and advice. We can teach you to use these tools and apply them to your project, as well as help you to find others who may assist with technologies outside of our scope. Our assistance can be as deeply involved or peripheral as you want, and any intellectual property is owned by the developer. What follows is an introduction to Photolithography at CNF, and to some of the technologies available here. The discussions are general, with some of it more specific to the CNF tool set. More information is available from the staff associated with any specific area of the facility. 6 Lithography Basics Manufacture of devices depends on selective processes: Removal of material -- Etching Addition of material -- Deposition Modification of material -- Implantation, diffusion, etc. Defined areas of the substrate must be protected from or exposed to these processes. These areas become the pattern for one layer of the device. Pattern definition takes place in the resist -- a thin layer of polymeric material that is usually spin-coated onto the substrate. The resist is then modified so that it remains in some areas and is removed in others. This is a two-step process: Exposure -- Incident radiation, particles Development -- Selective removal in solvent or base liquid TYPES OF EXPOSURE Light -- 436 nm - 157 nm: Near-UV to Deep-UV Optical Lithography EUV -- 13.5 nm: EUV Lithography X-rays -- 5 nm - 0.4 nm: X-ray Lithography Electrons -- 10 keV - 150 keV: Electron Beam Lithography Ions -- 50 keV - 200 keV: Focused Ion Beam Lithography METHODS OF EXPOSURE Direct Write -- Electrons, ions, or photons are focused onto a small diameter spot which is scanned directly onto the resist; this is a serial exposure process. Masked Exposure -- Light, EUV, or X-rays are imaged onto the resist through a mask; this is a parallel exposure process. 7 Serial Exposure Parallel Exposure 8 DEVELOPMENT Exposure causes a physical or chemical change in the resist. Different mechanisms are responsible for these changes in the various types of resist. Development takes place in a liquid base or solvent, depending on the resist type. In general, resists can be either: Positive -- exposed areas become more soluble in the developer; they are removed by development Negative -- exposed areas become less soluble in the developer; they remain after development After development is pattern transfer (etching, deposition, implantation, etc.). 9 Lithography at CNF CAD Direct Mask Write Making JEOL JEOL NABITY DWL2000 DWL66fs DWL2000 6300 9500 E-beam Laser GCA ASML GCA Suss Suss ABM 5X i-line 4X 248nm 5X g-line MJB4 MA6 8” Steppers 6” Stepper Contact aligners 10 SOME SUGGESTIONS The most difficult thing about lithography is that you must know what you want to accomplish and how you will do it before you design the lithography plan. In particular, you have to think about: Your pattern requirements The requirements of the lithography tools The requirements of the techniques you will use for pattern transfer These requirements will be discussed going forward. HERE ARE SOME SUGGESTIONS BEFOR GETTING STARTED: 1. Think about what type of device you want and how to implement it. 2. Gather information from this manual, staff members, and other researchers about the best tools and techniques to use before you begin to design the pattern. 3. Design the pattern using the information you have gathered paying careful attention to the requirements listed above. 4. Perform lithography, pattern transfer, etc. 5. Repeat steps 1 - 4 as many times as necessary to get it right. 11 HOW CNF WORKS: (A Staff Member's Perspective) Your (usually) friendly local CNF staff member is balancing the requirements of local users, outside users, machine maintenance, process characterization, materials supply, and many other things. So, please keep in mind: The more thinking and preparation you do, the more specific the questions you ask will be, and the more time you will end up saving yourself and the staff member. The more advance notice you can give about when you would like to talk about your process or be trained on equipment, the better. The more responsible you can be around the lab, the less we have to clean up after you, and the more time we have for answering your questions. And, last but not least, please be patient! 12 OPTICAL LITHOGRAPHY TECHNIQUES OPTICAL RESISTS The resist system used almost universally for UV (436-365nm) photolithography is the DNQ Novolak system: novolak resin with a diazonaphthoquinone sensitizer. The basic forms for the resin and sensitizer are shown below: Moreau, p. 32. 11 The novolak resin is rendered base-insoluble by the addition of the sensitizer, or photoactive